1. Field of the Invention
The present invention pertains to the field of transducers, and more particularly to transducer backing materials and methods of applying backing materials to transducers.
2. Background
Piezoelectric transducers find a wide variety of application in ultrasonic and electroacoustic technologies. Characterized by the presence of a shaped, piezoelectric material such as, for example, lead zirconate titanate (PZT), these devices convert electric signals to ultrasonic waves, and generally vice versa, by means of the piezoelectric effect in solids. This effect is well known in the art of transducers and their manufacture. A piezoelectric material is one that exhibits an electric charge under the application of stress. If a closed circuit is attached to electrodes on the surface of such a material, a charge flow proportional to the stress is observed. A transducer includes a piezoelectric element, and if necessary, an acoustic impedance matching layer, or multiple matching layers, and an acoustically absorbing backing layer.
Transducers can be manufactured according to conventional methods. Thus, a thin piezoelectric transducer element is metalized on its two surfaces with a conductive coating such as, for example, gold plating over a chrome layer. The thickness of the piezoelectric element is a function of the frequency of sound waves. One surface of the piezoelectric element can be coated with an acoustic impedance matching layer, or multiple matching layers, as desired. A backing layer may be attached to the backside of the piezoelectric element. The backing layer material is typically cast in place via a mold such that the piezoelectric element lies between the matching layer and the backing material. The matching layer, which may be formed of an electrically conductive material, serves to couple between the acoustic impedances of the piezoelectric element and the material targeted by (i.e., at the front of) the transducer. Individual piezoelectric transducers are machined from the piezoelectric-material/matching material-layer.
An ideally characterized piezoelectric transducer would transmit 100% of the ultrasonic radiation to the front of the transducer, and no ultrasonic waves to the back. It is desirable, therefore, to use a lossy material for the backing layer. A conventional backing material, for example, is an encapsulate, soft gel containing tungsten, which is known in the art to serve as an acoustic absorber. According to conventional application methods, the backing material is pressurized to about 12,000 psi. The pressurization squeezes out excess gel and gives rise to a high-density encapsulate gel with enhanced concentration of tungsten. However, even with pressurization, inconsistent electrical conductivity from lot to lot, or within a given lot, can result because the tungsten concentration is still not high enough to maintain series contact between the tungsten particles across the backing material.
To enhance electrical conductivity, flakes of silver can be added to the backing-material mix. However, the gel, which is a relatively nonsticky substance, is generally rendered less effective in adhering the piezoelectric layer to the backing layer. Consequently, manufacturing yields can decrease because a higher proportion of individual transducers may have their tops sheared off during the production process. In addition, pressurization causes inconsistent densities across a given backing material. Therefore, the acoustic impedance (the product of the density and the speed of sound) varies across the backing material, resulting in individual transducers with widely divergent characteristics. Moreover, the pressurization necessitates a long cure time for the backing material. Thus, there is a need for a backing material and application process that improve yield consistency, reduce manufacturing time, and produce more efficient transducers.